This invention relates generally to EDM (electrical discharge machining) apparatus and is specifically concerned with a new and improved electronic depth control for such apparatus. In certain respects the present invention may be considered as an improvement upon the earlier commonly assigned patent application entitled "Electronic Depth Controller For EDM Apparatus", Ser. No. 346,689, filed Feb. 8, 1982, now abandoned.
EDM has been successfully applied to machining of precision holes in objects. The advantages of EDM are wellknown in this respect and will not be repeated here. Suffice it to say the EDM can achieve precision results where other procedures and apparatus would be totally incapable of even approximating the performance of EDM. In spite of the manifold advantages of EDM, there are particular products where extreme precision in machining holes is required. An example of one such situation is the machining of multiple blind holes in a workpiece where the hole depths are critical. Another example is where multiple through-holes are being machined, and it is important that all electrodes break through at substantially the same time.
Advances in EDM control apparatus are demonstrated by assignee's control system identified by the designation CP-60. The CP-60 control system is a microprocessor-based control which greatly enhances the performance of EDM apparatus. That system represents an advance toward the ultimate objective of a completely adaptive control system. In an adaptive control system actual machining conditions, or indicators of actual machining conditions, are monitored while in progress, and the manner of control is concurrently adapted in respect of the monitored conditions or indicia thereof so that optimum machining action occurs. By way of example an adaptive control may on the one hand provide a more rapid machining procedure where the actual machining is easier than anticipated, and it may on the other hand provide a slower machining rate where the actual machining operation is more difficult than originally anticipated.
The above mentioned commonly assigned patent application, Ser. No. 346,689 relates to an electronic depth controller which is employed in operative association with the main EDM control system. The electronic depth controller acts upon selectable input data relating to advance of the machining electrode or electrodes, and hence depth of hole or holes, machined in a workpiece. It interacts with the main EDM control system in a manner whereby difficult hole machining problems such as those referred to above in connection with blind holes and simultaneous break through of multiple holes are solved. This can have important benefits in the EDM machining of workpieces. By virtue of the electronic character of the depth controller, in conjunction with the electronic character of the main control system, virtually instantaneous response to actual machining conditions occurs whereby depth control and simultaneous breakthrough are accomplished with precision. In this regard the preferred embodiment of depth controller comprises its own microprocessor which receives selectable input data from input selector switches which are mounted on a control panel. The microprocessor acts upon data supplied by the selectable input switches and provides output data based upon the selectable input data which is conveyed to the microprocessor of the main control. The electronic depth controller thereby provides information relating to machining depths and the main control acts upon this information in controlling the machining process. The microprocessor of the depth controller is programmed with its own set of operating instructions. The input selector switches on the control panel include one set of switches for setting an initial depth limit, another set of switches for setting a machining increment, and another for setting the number of machining increments. It also possesses the capability for operating in either a mode 1 or a mode 2 format.
The present invention is directed to a new and improved electronic depth controller for EDM apparatus in which the overall operating efficiency is improved and which automatically esatblishes the correct zero reference point for the initiation of EDM machining and thereafter terminates EDM machining operation once the desired depth limit has been reached. An advantage of this aspect of the invention is that it becomes unnecessary to gage a workpiece to a reference plane for purposes of determining the zero depth reference. Instead with the present invention the zero reference plane is established automatically by the depth controller itself. This reduces the time spent by a machine operator in setting up a job on a machine. Also by minimizing operator involvement it reduces the likelihood for operator-induced errors.
The invention has the further advantage in that maximum efficiency in the operating cycle can be obtained because the electrode is fed toward the workpiece at the maximum rate of advance until such time as actual machining occurs. Upon occurrence of actual machining the servo drive is automatically slowed down from the rapid advance rate to a slower machining feed rate. The depth controller of the present invention detects the sudden change in velocity of the servo due to the detection of initial machining current and it uses this sudden change in velocity of the servo to set the zero reference plane from which the machining depth is measured. Thus the zero reference plane is always properly defined as the surface of the workpiece in which the machining takes place. The absolute location of the zero reference plane is immaterial.
The foregoing features, advantages, and benefits of the invention along with additional ones, will be seen in the ensuing description and claims. The drawings disclose a preferred embodiment of the invention in accordance with the best mode contemplated at the present time in carrying out the invention.